Saudi King Salman has ordered the establishment of specialised departments in the public prosecutor's office to investigate and prosecute corruption cases, the government's information office said in a statement on Sunday.

The move is intended to increase effectiveness and accelerate the process of combating corruption, the statement quoted Attorney General Sheikh Saud al-Mujib as saying.

Saudi authorities detained hundreds of top businessmen and royals in November and held them for several months at the Riyadh Ritz-Carlton in a sweeping anti-corruption probe.

Most detainees have since been released after reaching financial settlements with the government, but several dozen others remain in custody and may stand trial.

Officials have not released the names of those still held, the allegations against them or plans for how the cases could be prosecuted.

In early March, Forbes magazine excluded all Saudi Arabian tycoons from its annual list of the world's richest people.

Forbes said earlier in the week that it was "impossible to know definitively who gave how much to whom when".

As a consequence, the magazine said it had removed the 10 Saudi billionaires who made the cut last year, including Alwaleed bin Talal, whose wealth was previously valued at $18.7bn, and Mohammed al-Amoudi, who was said to have a $8.1bn personal fortune.

This left Nassef Sawiris, of the Sawaris family dynasty, ranked as the richest man in the Middle East with a personal fortune of $6.6bn. Four of his family members are also ranked on the billionaires list.

"With greater clarity regarding their wealth, some might eventually return to the ranking," Forbes said.

The magazine's Middle East edition said in a separate statement on Thursday that the wealth of Saudi billionaires was believed to have increased from $42.1bn last year due to the rise in oil prices and capital markets globally, but would be excluded due to the reported asset seizures.